The present invention is related to a column unit for an electric power steering apparatus comprising a steering column for supporting a steering shaft on the inside thereof such that the steering shaft can rotate freely, and a housing that houses component parts of the electric power steering apparatus inside thereof.
As illustrated in FIG. 45, a steering apparatus for an automobile is constructed so as to apply a steering angle to the front wheels by pushing or pulling a pair of left and right tie rods as rotation from the steering wheel 1 is transmitted to the input shaft 3 of a steering unit 2 and rotates the input shaft 3. The steering wheel 1 is supported by and fastened to the rear end section of a steering shaft 5, and with this steering shaft 5 inserted in the axial direction through a cylindrical shaped steering column 6, the steering shaft 5 is supported by the steering column 6 so as to be able to rotate freely. In an electric power steering apparatus, the front end section of the steering column 6 connected and fastened to the rear end section of a housing 9 that houses a worm reducer 7 and a torque sensor (displacement sensor) 14 (FIG. 31) which constitutes a torque measurement apparatus 8, that form an electric assist mechanism (steering assist unit). Moreover, an electric motor 10, which functions as a power source for the electric assist mechanism, is supported by and fastened to the housing 9.
When the steering shaft 5 is rotated by the steering wheel 1, the torque measurement apparatus 8 measures the direction and size of the torque that is applied to the steering shaft 5. This torque measurement apparatus 8 is supported inside the housing 9 so that it can rotate freely, and comprises an input shaft 12 and output shaft 13 that are connected together by a torsion bar 11, and a torque sensor 14 for measuring the amount of relative displacement in the direction of rotation between the input shaft 12 and output shaft 13. Based on the measurement results from this torque measurement apparatus 8, the electric motor 10 applies assist torque in the same direction as the operating direction of the steering wheel 1 to the output shaft 13 by way of a worm and a worm wheel which is provided on the output shaft 13 and engages with the worm, and the causes the output shaft 13 to rotate with a torque larger than the torque inputted to the input shaft 12 from the steering shaft 5.
The tip end section of the output shaft 13 is connected to the rear end section of an intermediate shaft by way of a universal joint 15a, and the front end section of this intermediate shaft 16 is connected to the input shaft 3 by way of a separate universal joint 15b. The construction illustrated in FIG. 45 incorporates both a tilt mechanism for adjusting the up/down position of the steering wheel 1, and a telescopic mechanism for adjusting the forward/backward position. Therefore, the middle section of the steering column 6 is supported by an upper support bracket 18 that is supported by the vehicle body 17 such that adjustment of the up/down position and the forward/backward position is possible. In order to construct this tilt mechanism, a support cylinder 19 that is provided on the top end section on the front side of the housing 9 is supported by the vehicle body 17 by way of a lower support bracket 23 so that tilting about the horizontal axis is possible. Moreover, in order to construct the telescopic mechanism, the steering shaft 5 is a combination of an inner shaft and an outer shaft that are capable of transmitting torque and can be expanded or contracted, and the steering column 6 is a combination of an outer column and an inner column that can be expanded and contracted.
In a column unit for an electric power steering apparatus comprising a combination of this kind of steering column 6 and housing 9, In order to lighten the weight and reduce the cost, manufacturing the housing 9 using synthetic resin is disclosed in JP2009-298246(A). FIG. 46 illustrates an example of conventional construction of the column unit for an electric power steering apparatus disclosed in JP2009-298246(A). In the case of this conventional construction, a sensor housing 20 for housing the torque measurement apparatus 8, and a gear housing 21 for housing the worm reducer 7, both being manufactured using a synthetic resin such as polyamide resin, are welded to form a hollow housing 9a. In this example, the steering column 6a comprises an outer column and an inner column that can be expanded and contracted, however, in this kind of construction, when the housing 9a is made of synthetic resin, from the aspect of maintaining the strength and rigidity, the steering column 6a may have to be formed using a metal such as a ferrous alloy. In the case of the conventional construction illustrated in FIG. 46, by fitting the front end section of the metal cylindrical shaped steering column 6a around the outside of a cylindrical section 22 that is formed on the rear end section of the housing 9a made of synthetic resin, the rear end section of the housing 9a is connected and joined with the front end section of the steering column 6a. 
However, in the case of this conventional construction in which the housing 9a made of synthetic resin is combined with a metal steering column 6a in this way, increasing the bonding strength between the rear end section of the housing 9a and the front end section of the steering column 6a is difficult. In other words, even when the rear end section of the housing 9a and the front end section of the steering column 6a are fitted and fastened together by an interference fit, there is a possibility that the bonding strength of this fastened and joined section will gradually decrease due to a difference in linear expansion coefficient of the synthetic resin of the housing 9a and the metal of the steering column. Particularly, when the degree of expansion of the outer diameter of the cylindrical section 22 becomes larger than the degree of expansion of the inner diameter of the front end section of the steering column 6a, the pressure (surface pressure) holding the outer circumferential surface of the cylindrical section 22 becomes extremely large, and the cylindrical section 22 plastically deforms in a direction such that the outer diameter becomes smaller. As a result, the drop in the bonding strength with this fastened and joined section becomes very large.
Moreover, the strength and rigidity of this fastened and joined section may not be large from the initial stage due to the interference fit between metal and synthetic resin. Therefore, even when a force in the bending direction is applied to this fastened and joined section when adjusting the height of the steering wheel 1, there is a possibility that the outer circumferential surface of the cylindrical section 22 will plastically deform, and the bonding strength of this fastened and joined section will drop. In either case, when this bonding strength decreases, there is a possibility that a problem will occur of movement or vibration of this joined section between the steering column 6a and housing 9a. A similar problem may also occur when the steering column is made of a metal such as a ferrous alloy having a specified amount of strength and rigidity, and the housing is made of a metal such as an aluminum alloy that is lightweight but is soft compared to a ferrous alloy.
On the other hand, in order to protect the driver during a collision accident, a mechanism for absorbing impact during the collision is provided in the steering apparatus for an automobile. First, the intermediate shaft 8 is made so as to be able to contract along the entire length due to an impact load, such that, during a primary collision when the automobile collides with another automobile, even though the steering unit 2 displaces toward the rear, the steering wheel 1 is prevented from displacing in the rear direction by way of the steering shaft 5 so as to apply heavy pressure to the body of the driver. After the primary collision occurs, a secondary collision occurs when the body of the driver collides with the steering wheel 1, however, in the case of this collision, the driver is protected with the steering column 6 which supports the steering wheel 1 supported against the vehicle body so as to be able to break away in the forward direction due to an impact load in the forward direction that occurs during a secondary collision, and with an energy absorbing mechanism that absorb this impact load.
In this kind of impact absorbing steering column unit as well, from the aspect of reducing the weight and lower the cost, efforts are being made to reduce the weight of the steering column itself. FIG. 47 illustrates an example of a column unit for an electric power steering apparatus that is disclosed in JP2010-36677(A). In this construction, the outer column 24 of the steering column 6b comprises a support cylinder 26 that is made of metal and a sleeve 27 that is made of synthetic resin, and a metal inner column 25 of the steering column 6b fits around the inside of the inner circumferential surface of the sleeve 27. Moreover, a plurality of protrusions 28 are formed in the portion on the inner circumferential surface on the rear end section of the sleeve 27 where the inner column 25 does not normally fit, and impact energy is absorbed when the inner column 25 scrapes off these protrusions 28. In this construction, by lightening the weight by making the sleeve 27 of the outer column 24 using synthetic resin, and providing an energy absorbing mechanism inside the steering column 6b, the impact absorbing column unit can be made more compact and the stable impact energy absorbing characteristics are achieved.
However, in the apparatus disclosed in JP2010-36677(A), the sleeve 27 of the outer column 24 is formed using synthetic resin, however weight is not sufficiently reduced. Furthermore, there is a need for not only reducing the overall weight of the unit, but in studying improvement thereof, a need arose to maintain a specified amount of strength and rigidity of the impact absorbing steering column, and further more to maintain the installation rigidity between the steering column and the vehicle body during a secondary collision.